1. Field of the Invention
This invention relates generally to data storage device servo control systems and, more particularly, to demodulation of servo information for positioning the heads of data storage devices across the surface of a moving storage medium.
2. Description of the Related Art
In conventional computer data storage systems having a rotating storage medium, such as a magnetic or magneto-optical disk, data is stored in a series of concentric or spiral tracks across the surface of the disk. The data comprises a series of variations in disk surface magnetic orientation recorded laterally in the tracks. A magnetic read/write head suspended over the disk surface transduces the variations in magnetic orientation and produces a readback signal. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The magnetic read/write head detects the variations in magnetic orientation and generates the readback signal as the disk rotates relative to the read/write head at thousands of revolutions per minute.
Reading and recording data in a desired one of the tracks requires knowledge of the read/write head position relative to the desired track as the disk rotates and requires precise centering of the read/write head over the track. Conventionally, the read/write head is mounted on a disk arm that is moved by a servo. A disk drive servo control system controls movement of the arm radially across the disk to move the read/write head from track to track and, once over a selected track, to maintain the head in a path centered over the track in a track-following operation.
A servo control system maintains the read/write head centered over a track by reading servo information from the disk surface. The servo information comprises a pattern of high-frequency magnetic flux transitions, generally flux reversals, that are pre-recorded in the tracks at the time of disk manufacture. A servo read head, which can be either the same head used for reading the binary data or can be a dedicated servo head, transduces the servo information and produces a servo signal. The servo signal includes a digital portion that provides the number of the track from which the servo pattern was read and an analog portion. The analog servo signal is a signal the magnitude of which indicates the position of the read/write head relative to the track centerline. The analog signal is used to generate a position error signal and thereby control the disk arm servo. Thus, the servo control system detects the track over which the read/write head is positioned and controls movement of the head relative to the track.
A common method for providing servo information to a disk servo control system is known as the sector servo method. In the sector servo method, each disk surface of a disk drive includes both servo information and binary data within a single track. The tracks on a sector servo disk surface are divided into radial sectors comprising short servo information fields interspersed among data fields. Each servo information field includes a sector marker, which indicates to the read/write head that servo information immediately follows in the track, a track identification number, and a high-frequency servo burst pattern. The sector servo method advantageously provides design efficiencies in that a single read/write head is used to read the servo information and also to read and record data from the disk. In addition, less of the total disk surface area is used for servo information as compared with other designs, such as those using a dedicated servo head. In this way, the sector servo method increases the storage media surface area available for recording data.
The servo burst portion of the head readback signal is used to generate a corrective position error signal (PES) to cause a head positioning servo to move and control the position of the read/write head over the disk. The PES provides an indication of the direction and extent of read/write head movement required to maintain the head centered about the track. Servo patterns vary depending on the manufacturer and the system. For example, in one system a first group of transitions comprises a first servo pattern burst and a second group of transitions comprises a second servo pattern burst. The PES is produced, or demodulated, from the servo bursts by determining the magnitude of each burst and then determining the amplitude difference of the first and second burst magnitudes. If the amplitude difference is zero, then the read/write head is positioned exactly over the track centerline. A positive amplitude difference indicates that the head is off center in one direction and a negative amplitude difference indicates that the head is off center in the opposite direction. Other patterns generate four servo bursts that comprise a quadrature PES.
With conventional digital sampled servo signal demodulation systems, the analog servo signal must be digitized before being provided to a servo signal digital processor. An analog servo demodulator resides in a data channel chip module that processes the head readback signal. In a typical sector servo system with quadrature PES, the analog servo demodulator produces four analog PES voltages that are provided to an off-chip analog-to-digital converter that digitizes the quadrature signals and provides them to the servo signal digital processor. Conventional synchronous data channel architectures already contain a high-speed flash analog-to-digital (A/D) converter internally in the data channel chip module. The flash A/D converter is also known as a parallel-comparator A/D converter. The data channel flash A/D converter samples the data portion of the head readback signal according to a predetermined sampling clock and produces a digitized signal for data channel processing. Flash A/D converters are relatively complex and costly but provide the performance necessary for high-speed data channel architectures. It would be advantageous to use the internal flash A/D converter of the data channel and the associated high-speed logic to demodulate and digitize the servo signal as well as the data information, thereby eliminating additional expense and complexity of dedicated analog PES demodulators and off-chip servo signal A/D converters.
Conventionally, sector servo demodulation circuits that demodulate and digitize the servo signal with the data channel flash A/D converter use architectures that require the sampling rate of the converter during servo signal processing to be an integer multiple of the fundamental frequency of the servo burst pattern. For example, if a quadrature servo pattern contains four 5.0 MHz flux transition burst cycles, then the A/D converter sampling rate would be 40 MHz to obtain eight samples per burst cycle. The servo burst pattern frequency remains constant over the entire disk surface. Therefore, the sampling rate for digitizing the servo signal must remain constant over the entire disk surface for proper processing of the servo information.
Many disk storage devices are of a banded data format. On a disk with banded data, the frequency of the recorded magnetic flux transitions that comprise the binary data will be changed depending on the radial distance of the read/write head from the disk outside diameter, or disk circumference. In particular, the recording frequency of the transitions in terms of disk revolutions is greater toward the disk circumference as compared to the recording frequency toward the disk center. For example, approximately six data bands typically are distributed across the surface of a standard size 31/2 inch floppy diskette. Data is recorded in each data band at a different frequency from adjacent bands to permit the number of data bits per linear distance in a track to be maintained at a relatively high value regardless of the track distance from the disk center. It should be apparent that there is more total surface area in a track at the disk circumference than at the disk center and that more data can be recorded per disk revolution at the disk circumference than at the disk center. The use of banded data disk storage systems has become more common as users demand greater storage capacities from disk systems.
In a data storage disk having banded data, the frequency of the sampling clock of the data channel flash A/D converter must change with each different data band. A data channel clock synthesizer generates the proper sampling clock frequency as a function of each data band. Identification of the data band can be obtained from the sector servo fields. Because the servo burst pattern frequency is maintained constant over the entire disk surface, the source for the sampling clock of the servo signal cannot come directly from the data channel A/D converter clock synthesizer. Therefore, banded data disks in which a single flash A/D converter is shared between the data channel and servo signal demodulator generally require the servo signal sampling clock signal to come from an external source that must be switched in to the flash A/D converter when the read/write head is reading the sector servo data field. The necessity of a separate clock generation circuit adds another frequency source to the data channel chip module, increasing complexity and cost of the module.
From the discussion above, it should be apparent that there is a need for a sampled servo signal demodulation system for banded disk drives that permits a data channel flash A/D converter to be used for both data channel decoding and servo signal demodulation without need for separate sampling clock circuits nor for switching between such clock signal sources. The present invention satisfies this need.